This invention relates to a magneto-inductive flow-measuring method for moving fluids, whereby the magnetizing field coils generating the magnetic field are energized in gapped fashion and each measuring period encompasses a positive half-cycle of the magnetic field and a negative half-cycle of the magnetic field.
The fundamental operating principle of a magneto-inductive flowmeter for moving fluids goes all the way back to Faraday who in 1832 suggested employing electrodynamic induction for measuring flow rates. According to Faraday""s law of induction, an electric field intensity perpendicular to the direction of flow and to the magnetic field is generated in a flowing medium containing charge carriers and passing through a magnetic field. In a magneto-inductive flowmeter this law is applied by using a magnet, generally consisting of two magnet coils, which generates a magnetic field perpendicular to the flow direction in the measuring tube. Within this magnetic field, each volume element traveling through the magnetic field of the moving fluid contributes its built-up field intensity to the measuring voltage collected by way of the test electrodes. In conventional magneto-inductive flowmeters, these test electrodes are designed for either galvanic or capacitive coupling with the moving fluid. One particular characteristic of magneto-inductive flowmeters is the proportionality between the measuring voltage and the flow rate of the fluid, averaged across the cross section of the pipe, i.e. between the measuring voltage and the volumetric flow.
The magnetic field of a magneto-inductive flowmeter is generated by feeding an energizing current to the two magnet coils. This approach is susceptible to measuring-signal errors due to a less than ideal energizing current and to irregularities in the flowing medium itself. The latter are attributable for instance to electrochemical interference caused by reactions between the electrodes and the moving fluid. Typically encountered in situations of this nature are low-frequency interference signals overlapping the measuring signal. In traditional magneto-inductive flowmeters operating in a continuous mode, i.e. where the excitation current energizing the magnet coils alternates between positive and negative without any intervals between the positive and negative half-cycles, these low-frequency interference signals affecting the measuring signals can be compensated for by the interpolation of a simple filter which is capable of suppressing linear drifts superposed on the measuring signal.
For example, to permit the operation of a battery-powered magneto-inductive flowmeter it may be desirable to minimize the overall energy consumption of the magneto-inductive flowmeter. Most of the energy is used up by the field coils as they generate the magnetic field. To address that problem, U.S. Pat. No. 4,766,770 proposes gapped energizing of the field coils generating the magnetic field. The excitation current and thus the measuring signal are composed of cycles or periods with a positive and a negative half-cycle, with the individual periods separated by idle intervals during which no energizing takes place. A measured value can be delivered only after a full period since each such value must consist of at least two half-cycles. However, the last half-cycle of a period is separated too far from the first half-cycle of the next-following period by the intervening idle interval, to allow for the evaluation of a measuring signal based on these two half-cycles.
The measured value associated with the two half-cycles of a measuring period is defined as the value that represents the area covered by the measuring signal relative to the zero line. The selection of the zero line relative to the measuring signal is not critical for as long as the area covered by the positive half-cycle is evaluated as positive and the area covered by the negative half-cycle is evaluated as negative. For simplicity""s sake, the assumptions here and in the following description are based on a linear interference which at least in a first approximation corresponds to the interferences encountered in an actual measuring operation. However, in determining the measured value, such linear interference signals heterodyned over the measuring signal pose the following problems:
In the case of a signal with an interference that has a negative slope, the area values below the curve of the measuring signal are somewhat larger than those of a clean signal. The opposite is the case for a signal superposed by an interference with a positive slope, where each of the areas covered by a half-cycle is too small by a certain value.
It is the objective of this invention to introduce a magneto-inductive flow-measuring method which saves energy while at the same time permitting simple and precise determination of the measuring signal even when the measuring signal is overlapped by an interference signal, as well as a magneto-inductive flowmeter serving that purpose.
According to a first conceptual aspect of this invention, the problem first above mentioned is solved by providing for the measuring periods to include an area before each first half-cycle of the magnetic field and an area after the last half-cycle of the magnetic field, by using the field intensities measured therein for quantifying an interference signal superposed over the measuring signal, and by correcting the measuring signal correspondingly. The extra signals in the areas preceding the first half-cycle and, respectively, following the last half-cycle would have to be zero since no magnetic field regenerated that would induce a field intensity of the flowing medium, so that any extra, non-zero signals actually captured due to the drift overlay over the measuring signal provides the information on the interference which can be used for appropriately adjusting the measuring signal.
A particularly simple and effective correction offers itself by virtue of the fact that the measuring periods are, in each case, composed of exactly one positive half-cycle of the magnetic field and exactly one negative half-cycle of the magnetic field, that during the positive half-cycle of the magnetic field and during the negative half cycle of the magnetic field the measuring signal is upslope-integrated, producing the measured subvalues U1 and U2, respectively, that during a time span corresponding to the duration of a half-cycle directly before the first half-cycle of the magnetic field and corresponding to the duration of a half-cycle directly following the last half-cycle of the magnetic field is upslope-integrated, producing the measured subvalues U1a, and U2a, respectively, and that the measured value W associated with a given measuring period is calculated according to this equation:
W=U1xe2x88x92U2xe2x88x92⅓ (U1axe2x88x92U2a)
A particularly simple transition into a possibly desired continuous measuring process is obtainable when the measuring periods always begin with the same half-cycle, i.e. either always with the positive half-cycle or always with the negative half-cycle.
According to another conceptual aspect of this invention, the problem first above mentioned is solved in that the measuring periods always include the same, even number of half-cycles, that the consecutive measuring periods begin in alternating fashion with a positive half-cycle of the magnetic field or, respectively, a negative half-cycle of the magnetic field, and that the measuring signal is determined by averaging the values measured during the consecutive measuring periods. Since in this case the measured value thus determined will be slightly too high whenever the measuring period begins with a positive half-cycle, and slightly too low whenever the measuring period begins with a negative half-cycle, the interference superposed on the measuring signal will be essentially averaged out as a function of time.
The method described above is made particularly simple and effective in that the measuring periods consists, in each case, of exactly one positive half-cycle of the magnetic field and exactly one negative half-cycle of the magnetic field, that during the positive half-cycle of the magnetic field and during the negative half-cycle of the magnetic field of the measuring signal is, in each case, upslope-integrated into measured subvalues, and that for the determination of the measured value the series of values derived from the respective difference between the measured subvalues during the positive half-cycle of the magnetic field and, respectively, during the negative half-cycle of the magnetic field is subjected to low-pass filtering.
According to another conceptual aspect of this invention, the problem first above mentioned is solved in that the measuring periods consist in each case of an odd number of half-cycles of the magnetic field and the measured value is averaged in each measuring period. When in a given measuring period all consecutive half-cycles, i.e. always one positive half-cycle together with one negative half-cycle, are used for determining a measured subvalue through averaging, an odd number of half-cycles in a measuring period will result in exactly as many subvalues being determined as too large, meaning those where the negative half-cycle follows the positive half-cycle, as subvalues are determined as being too small, meaning those where the positive half-cycle follows the negative half-cycle. Overall, as a result, all of the interference signal superposed over the measuring signal will essentially be identified by averaging.
The method described above is made particularly simple and effective in that the measuring periods are each composed of exactly three half-cycles of the magnetic field, that during the three half-cycles of the magnetic field the measuring signal is upslope-integrated, producing measured subvalues U1, U2 and U3, respectively, and the measured value W associated with any given measuring period is calculated according to the equation
W=0,5 (U1xe2x88x922U2+U3)
if the measuring period includes two positive half-cycles, or it is calculated according to the equation
W=0,5 (xe2x88x92U1+2U2xe2x88x92U3)
if the measuring period includes two negative half-cycles. In the process a particularly simple transition offers itself into a possibly desired continuous measuring procedure when the measuring periods begin alternatingly with the positive half-cycle or with the negative half-cycle, respectively.
Another object of this invention is a magneto-inductive flow-measuring method whereby the field coils generating the magnetic field are continuously energized. According to the invention, the characteristic feature of this method lies in the fact that the energizing current fed to the field coils is controlled in such fashion that during a full cycle, it attains a predetermined maximum positive value and a quantitatively identical predetermined maximum negative value, that the predetermined maximum positive value and the predetermined maximum negative value of the excitation i.e. energizing current are calibrated for the magnetic field to be produced, and that in a predetermined sequence at least two mutually different, calibrated energizing currents are employed. In this fashion and by virtue of the continuous operation of the magneto-inductive flowmeter, continuous measured values will also be available serving, for instance, to eliminate, by methods such as averaging, an interference signal overlaying the measuring signal, while at the same time the power consumption of the magneto-inductive flowmeter is reduced due to the fact that during its operation the energizing current is modified in the form of substantial intermittent reduction.
Yet modifying the energizing current during the operation of a megneto-inductive flowmeter poses a problem insofar as the relationship between the magnetic field and the energizing current is nonlinear. This would normally require the addition of expensive sensors for the magnetic field serving to regulate the energizing current at desired levels. According to this invention, however, at least two mutually different, calibrated energizing currents are employed. These energizing currents are controlled in a way as to arrive at a compromise between energy conservation on the one hand, i.e. using as often as possible the lowest energizing currents possible, while on the other hand obtaining the highest possible data density from the measuring signal so as to eliminate to the most effective level possible any superposed interference signals.
In both cases, i.e. when the field coils generating the magnetic field are energized in gapped fashion as well as for continuous operation of the field coils, the problem first above mentioned is also solved by a magneto-inductive flowmeter in which, by means of suitable parameters preferably s et through local operator intervention or by automatic adaptive recognition of the measuring conditions, one or the other of the methods offered by this invention is selectable.